It is known in the prior art to form emission-enhancing coatings on the surfaces of electron emitter structures of field emission devices. These prior art coatings are employed to improve the emission current characteristics of the field emission device. Typically, the electron emitters are Spindt-tip structures made from molybdenum, and the emission-enhancing coating is a metal that is selected for its low work function, which is less than that of the molybdenum. The surface work function of molybdenum is about 4.6 eV. Processes for forming electron emitter structures, such as Spindt tips, from molybdenum are well known in the art.
Prior art emission-enhancing coatings are known to be made from a pure metal selected from the following: sodium, calcium, barium, cesium, titanium, zirconium, hafnium, platinum, silver, and gold. Also known are emission-enhancing coatings made from the carbides of hafnium and zirconium. These prior art coatings are known to improve the emission current characteristics of field emission electron emitters. However, these prior art coatings suffer from several disadvantages. For example, many have high electrical conductivities, which can cause electrical shorting between the individual gate electrodes and between the gate electrodes and cathodes
However, prior art methods for depositing these emission-enhancing coatings typically include blanket depositions over the entire cathode plate. This results in the deposition of emission-enhancing material between the gate extraction electrodes, which extract electrons from the electron emitters. These methods are not suitable for the coating of electron emitter structures of selectively addressable arrays of field emitters, such as are employed in field emission displays. In one prior art method for coating electron emitter structures with cesium, electrical conduction between the gate electrode and the cathode is mitigated by carefully controlling the thickness of the cesium layer.
It is also known in the art to coat electron emitters with films made from diamond-like carbon (DLC). This prior art coating is similarly employed for the purpose of reducing the work function of the surface of the electron emitters. In one prior art method for coating electron emitters with DLC, the selective deposition of the emissive DLC material is achieved by first forming nucleation sites on the surfaces of the electron emitters The nucleation sites are formed by selectively implanting carbon ions into the surfaces of the electron emitter structures, and not between the gate electrodes. The cathode surface is then exposed to a reactant material, which preferentially reacts at the nucleation sites to form the DLC, thereby mitigating deposition of the coating material between gate electrodes. However, this prior art method for localizing the coating material at the electron emitters is limited to the formation of DLC.
Accordingly, there exists a need for an improved method for coating electron emitters of a field emission device, which is useful for a variety of emission-enhancing coating materials, which does not cause adverse electrical conduction between individual gate electrodes and between the gate electrodes and the cathode electrodes, and which allows for variability of the thickness of the emission-enhancing coating.